Overcoming limited common-mode range for USB systems

ABSTRACT

An intelligent level shifter may be added to adjust the voltage level on the data lines (D+ and D−) used for communications in USB systems, to address the issue of missing negative common-mode range as defined by the USB specification. The level shifter may be part of a port power controller that allows adaptive shifting of the signal level in accordance with the current levels drawn on the supply line by a device, for example during charging. The port power controller may be operated in systems enabled for battery charging, and may combine overcurrent sensing (current meter for VBus) and the routing of the D+ and D− lines (used for the battery charging protocol) into a single package. By varying the voltage levels on the D+ and D− data lines according to the drawn current levels, the performance of USB Hosts ports and USB Hub ports may be greatly increased.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to USB devices, and morespecifically to overcoming the limited common-mode range for USBsystems.

2. Description of the Related Art

The Universal Serial Bus (USB) was developed to offer PC users anenhanced and easy-to-use interface for connecting an incredibly diverserange of peripherals to their computers. The development of the USB wasinitially driven by considerations for laptop computers, which greatlybenefit from a small profile peripheral connector. Among the manybenefits of the USB is a reduction in the proliferation of cables thatcan affect even the smallest computer installations. In general, USB hasbecome the interface of choice for PCs because it offers users simpleconnectivity. USB eliminates the need to have different connectors forprinters, keyboards, mice, and other peripherals, and supports a widevariety of data types, from slow mouse inputs to digitized audio andcompressed video. In addition, USB devices are hot pluggable, i.e. theycan be connected to or disconnected from a PC without requiring the PCto be powered off.

The USB specification has seen various revisions, with the USB 2.0standard challenging the IEEE 1394 interface (“Firewire”) as theinterface of choice for high-speed digital video, among others. The USB3.0 standard, representing the second major revision of the USBstandard, specifies a maximum transmission speed of up to 5 Gbits/s (640MBbytes/s), which is over 10 times faster than the maximum speedspecified in the USB 2.0 standard (480 Mbits/s). The USB 3.0 standardalso features reduced time required for data transmission, reduced powerconsumption, and is backward compatible with USB 2.0. With theproliferating design of increasingly smarter, faster, and smallerperipherals, the On-The-Go (OTG) Supplement to the USB 2.0 Specificationwas also developed to address the growing popularity of the portableelectronic devices market. OTG devices typically do not require a PChost, and can communicate directly with each other. For example, a PDAmay act as a USB host with the capability to print directly to a USBprinter, while also acting as a USB peripheral to communicate with a PC.In general, designers are facing increasing pressure to design smallerand faster products in less time and at lower cost.

Present day USB devices that communicate with a host over USB includeUSB printers, scanners, digital cameras, storage devices, card readers,etc. USB based systems may require that a USB host controller be presentin the host system, and that the operating system (OS) of the hostsystem support USB and USB Mass Storage Class Devices. USB devices maycommunicate over the USB bus at low-speed (LS), full-speed (FS), orhigh-speed (HS). A connection between the USB device and the host may beestablished via a four-wire interface that includes a power line, aground line, and a pair of data lines D+ and D−. When a USB deviceconnects to the host, the USB device may first pull a D+ line high (theD− line if the device is a low speed device) using a pull up resistor onthe D+ line. The host may respond by resetting the USB device. If theUSB device is a high-speed USB device, the USB device may “chirp” bydriving the D− line high during the reset. The host may respond to the“chirp” by alternately driving the D+ and D− lines high. The USB devicemay then electronically remove the pull up resistor and continuecommunicating at high speed. When disconnecting, full-speed devices mayremove the pull up resistor from the D+ line (i.e., “tri-state” theline), while high-speed USB devices may tri-state both the D+ and D−lines.

The USB standard provides very stringent guidelines for the allowedcommon-mode voltage on the differential data lines (D+ and D−). On theother hand, newer specifications allow for battery charging using a USBport, which oftentimes results in currents that are much higher thaninitially specified for USB, producing higher voltage drops across theUSB setups. This can lead to the need for thicker cables and/or reducedcable length when trying to communicate across the USB while theattached USB device is charging. Such change in cabling needs may notallow full backward compatibility for existing cabling infrastructures.

Other corresponding issues related to the prior art will become apparentto one skilled in the art after comparing such prior art with thepresent invention as described herein.

SUMMARY OF THE INVENTION

In one set of embodiments, an intelligent level shifter may be added toboost and/or attenuate the voltage on both Universal Serial Bus (USB)data lines (D+ and D−) used for communications in USB systems, toaddress the issue of missing negative common-mode range as defined bythe USB specification. The level shifter may be part of a port powercontroller that allows adaptive shifting of the signal level inaccordance with the current levels on the supply voltage line (VBus)provided by the port power controller. The port power controller may beoperated in systems enabled for battery charging, and may combineovercurrent sensing and the routing of the D+ and D− lines (used for thebattery charging protocol) into a single package. By varying the voltagelevels on the D+ and D− data lines according to the current levels(drawn) on the power supply line, the performance of USB Hosts ports andUSB Hub ports may be greatly increased.

Thus, in one set of embodiments, a port power controller (PPC) mayinclude a supply bus to provide a supply voltage, a data bus to carrydata, and a level shifter to adjust a voltage level of data on the databus according to the value of the current drawn on the supply bus. Insome embodiments the level shifter may also be programmable to adjustthe voltage level on the data bus by different levels depending on thedirection of the data flow on the data bus. That is, the level shiftermay adjust the voltage level on the data bus by a first amount, e.g.according to the current drawn on the supply bus by a device, when datais transmitted to the device, and adjust the voltage level on the databus by a second amount when data is received from the device. The firstamount and the second amount may be independentlyprogrammable/controllable. The PPC may provide the supply voltage, andthe data with the adjusted voltage levels for transmission to a device.In some embodiments, the data bus includes a USB D+ data line and a USBD− data line. The PPC may include a measurement unit to measure thevalue of the current drawn on the supply bus, and may generate a controlsignal based on the measured value of the current drawn on the supplybus to control the level shifter to adjust the voltage level of the dataon the data bus. The PPC may generally be used with a data bus thatincludes a pair of differential data lines, with the level shifteradjusting the voltage level of the data on both data lines of the pairof differential data lines. In some embodiments, to address the issue ofa negative common-mode range, when adjusting the voltage level of dataon the data bus, the level shifter may increase the voltage level ofdata on both data lines of the pair of differential data lines by a sameamount.

A method for overcoming limited common-mode range on a differential databus may include providing a supply voltage over a supply bus to adevice, receiving data on an incoming differential data bus, adjusting avoltage level of the received data according to a current drawn on thesupply bus, and providing the adjusted voltage level data fortransmission to the device over an outgoing differential data bus. Inadjusting the voltage levels of the data, the method may further includemeasuring the value of the current drawn on the supply bus, generating acontrol signal based on the results of the measurement, and adjustingthe voltage level of the received data by providing the control signalto a level shifter, with the level shifter increasing the voltage levelof the received data according to the control signal.

In some embodiments, the data may be received on a pair of differentialdata lines that include a D+ data line and a D− data line, and thevoltage level of the data on the D+ data line and the data on the D−data line may be adjusted by an equal amount. In some cases theadjustment may include raising the voltage value of the signal toovercome negative common-mode limitations. The method may includetransmitting the supply voltage and the adjusted voltage level data overa USB to a USB device.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the present invention may be obtained when thefollowing detailed description is considered in conjunction with thefollowing drawings, in which:

FIG. 1 shows one embodiment of a USB Host system that includes a portpower controller with D+/D− level shifting capability;

FIG. 2 shows a more detailed partial block diagram of one embodiment ofthe port power controller of FIG. 1;

FIG. 3 shows a block diagram illustrating the USB voltage drop budgetwhen drawing 500 mA current on the supply bus;

FIG. 4 shows a block diagram illustrating the USB voltage drop budgetreferenced to source when drawing 1 A current on the supply bus;

FIG. 5 shows a block diagram illustrating the USB voltage drop budgetand signaling levels from the Host to the device when drawing 1 Acurrent on the supply bus;

FIG. 6 shows a block diagram illustrating the USB voltage drop budgetand signaling levels from the device to the Host when drawing 1 Acurrent on the supply bus; and

FIG. 7 shows a flow diagram of one embodiment of a method to overcomelimited common-mode range on a differential serial bus.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and will herein be described in detail. Itshould be understood, however, that the drawings and detaileddescription thereto are not intended to limit the invention to theparticular form disclosed, but on the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the present invention as defined by the appendedclaims. Note, the headings are for organizational purposes only and arenot meant to be used to limit or interpret the description or claims.Furthermore, note that the word “may” is used throughout thisapplication in a permissive sense (e.g., having the potential to orbeing able to in some embodiments), not a mandatory sense (i.e., must).The term “include”, and derivations thereof, mean “including, but notlimited to”. The term “coupled” means “directly or indirectlyconnected”.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates one embodiment of an exemplary USB system, in which aport power controller 120 may be coupled between a USB Host/USB Hub 202and a USB connector 204, which may be part of a USB device or anotherUSB Hub (not shown). Alternatively, in some embodiments port powercontroller 120 may be part of a USB host (e.g. a part of USB Host 202),in which case USB connector 204 may represent the USB output of the USBhost (i.e., the output of USB host 202). USB power port controller 120may include an intelligent level shifter 116 coupled in the path of theD+ and D− data lines entering port power controller 120, to potentiallyboost and/or attenuate the voltage levels on those data lines based oncurrent levels (i.e. the levels of the current flowing) on the VBusline, and on the direction of the data traffic on data lines D+ and D−.

Adding intelligent level shifter 116 in the path of data lines D+ and D−addresses a missing negative common-mode range that is defined by theUSB specification. The level shifter may provide adaptive shifting ofthe signal level in accordance with the levels of the current on theVBus line. Port power controller (PPC) 120 as defined for batterycharging enabled systems may combine the overcurrent sensing (currentmeter for VBus) and the routing of the D+ and D− lines (used for thebattery charging protocol) into a single package. Thus, port powercontroller 120 with level shifter 116 may be used to enhance any USBHost or USB Hub port.

As indicated in the USB 2.0 specification, a high-speed capabletransceiver receiver is expected to conform to the receivercharacteristics specifications called out in Section 7.1.4.1 of the USB2.0 specification when receiving in low-speed or full-speed modes. Ahigh-speed capable transceiver which is operating in high-speed mode“listens” for an incoming serial data stream with the high-speeddifferential data receiver and the transmission envelope detector.Additionally, a downstream facing high-speed capable transceivermonitors the amplitude of the differential voltage on the lines with thedisconnection envelope detector.

When receiving in high-speed mode, the differential receiver is expectedto have the ability to reliably receive signals that conform to theReceiver Eye Pattern templates shown in Section 7.1.2. of the USB 2.0specification. Additionally, it is a strongly recommended guideline todesign a high-speed receiver to have the ability to reliably receivesuch signals in the presence of a common-mode voltage component (VHSCM)over the range of −50 mV to 500 mV (the nominal common-mode component ofhigh-speed signaling being 200 mV). Low frequency chirp J and Ksignaling, which occurs during the Reset handshake, is expected to bereliably received with a common-mode voltage range of −50 mV to 600 mV.These specifications, however, do not address the problems that may becreated when charging devices at higher than the initial specified 500mA rating, and communicating with those devices.

According to the USB 2.0 specification, the voltage drop budget isdetermined from the following:

-   -   The voltage supplied by high-powered hub ports is 4.75 V to 5.25        V.    -   The voltage supplied by low-powered hub ports is 4.4 V to 5.25        V.    -   Bus-powered hubs can have a maximum drop of 350 mV from their        cable plug (where they attach to a source of power) to their        output port connectors (where they supply power).    -   The maximum voltage drop (for detachable cables) between the        A-series plug and B-series plug on VBUS is 125 mV (VBUSD).    -   The maximum voltage drop for all cables between upstream and        downstream on GND is 125 mV (VGNDD).    -   All hubs and functions must be able to provide configuration        information with as little as 4.40 V at the connector end of        their upstream cables. Only low-power functions need to be        operational with this minimum voltage.    -   Functions drawing more than one unit load must operate with a        4.75 V minimum input voltage at the connector end of their        upstream cables.        FIG. 3 shows the values of the minimum allowable voltages in a        worst-case topology consisting of a Bus-powered Hub 304 driving        a Bus-powered Function 306, where the Bus-powered Hub 304 may be        controlled by Host (or Powered Hub) 302. The voltage values        shown in FIG. 3 are defined for the 500 mA limit.

However, considering the connectivity between Host 302 and Bus-poweredHub 304 and the corresponding voltage values, the voltage values changeas shown in FIG. 4, when the device (Function 306, via Bus-powered Hub304) draws, for example, 1 A (1 Amperes) in charging mode. As seen inFIG. 4, while the supply voltage range could be adjusted when drawing 1A in charging mode, data communication has to contend with a ground(GND) offset as high as 250 mV. As the encircled voltage values in FIG.4 indicate, when drawing 1 A, e.g. in charging mode, the GND offsetvalues double from 0.0015V to 0.030V, from 0.110V to 0.220V, and from0.125V to 0.250V, respectively. FIG. 5 shows the voltages seen on thedata lines D+ and D− (as an example) when the Host side 302 sends asignal at 400 mV on D+ and 0V on D− (referenced to the Host GND). The400 mV (0.400V) is seen as 150 mV on the device side (i.e. in this caseat Bus-powered Hub 304), and potentially 0V on the D− line (at Host side302), assuming that the negative voltage −250 mV is cut off withreference to the device side GND, which in this case is GND atBus-powered Hub 304. This results in a differential voltage of just 150mV, which is at the very limits of the USB specification. Communicationin the opposite direction is not affected, since the values fall withinthe allowable, high, positive common-mode range.

FIG. 6 shows communication from the Device side (i.e. from Bus-poweredHub 304) to the Host side 302. The GND offset adds to the signal,therefore the differential voltage does stay at the 400 mV level, with0V on the D− line. Accordingly, the voltages on the Host side are 650 mVon the D+ line and 250 mV on the D− line. It should be noted that thevalues shown in FIGS. 5 and 6 do not take into consideration any voltagedrop on the data lines D+ and D−.

One way to overcome the GDN offset is to simultaneously boost thetransmitted signal on both the D+ and D− lines on the Host side, whentransmitting data to the device. The boost may be related to the actualcurrent flow on the VBus line and the cable length. That is, the levelby which the signals are boosted may be related to the actual currentflowing in the VBus line. By relating the boost level to the actualcurrent, as opposed to merely setting a static boost level, overshootingthe allowed signal level—e.g. in case the current drawn by the device ismuch lower than expected—may be prevented. As shown in FIG. 2, a(battery charging) Port Power controller may integrate both the currentsensing on the VBus line and routing of the D+/D− lines, making itpossible to also integrate a flexible boost (and attenuation) mechanism.In one set of embodiments, a flexible boost factor with respect to theGND level on the device side may be implemented as an adaptive boostmechanism in PPC 120. The adaptive boost mechanism enables batterycharging on a much higher level while the device is in activecommunication, independent of additional cable length between thecharging port and the device.

FIG. 2 shows a more detailed block diagram of one embodiment of a PPC120 that includes an adaptive boost mechanism to adjust (increase ordecrease) the signal levels on the D+ and D− lines according to a numberof input factors, e.g. according to the actual current flow in the VBusline. A system supply voltage (V_(supply)) may be provided to PPC 120 atthe Vs pin, with PPC 120 providing a voltage output based on Vs(V_(supply)) at the VBus pin. That is, V_(supply) may be used as thepower source for the downstream devices. In some embodiments, V_(supply)may have a 5V value. In addition, a voltage Vdd may be provided at theVdd pin, to provide power to interface logic 110, and attach detector106. In some embodiments, Vdd may have a value of 3.3V. The D+ and D−data lines entering PPC 120 may run through a boost module 116, whichmay be controlled via a control signal 114 generated by Measurementblock 104. Measurement block 104 may operate Power Switch 102 to performcurrent measurements of the current on the VBus line, and generatecontrol signal 114 according to those current measurements. The boostedD+ and D− signal may be conditioned in Charger Emulator block 108, andthe conditioned boosted signals may be provided at the D+ and D−outputs.

In one set of embodiments, boost module 116 may be implemented as avoltage level shifter linked to the actual current drawn by the attacheddevice over the VBus line. As mentioned above, this link may beestablished by controlling the level shifter 116 from module 104, whichmay generate control signal 114 based on, among other things,measurements of the current on the VBus via power switch/amp meter 102.Module 104 may further generate control signal 114 based on a number ofother factors, for example according to programmable parameters relatingto the direction of data flow on data lines D+ and D−. Therefore, theboost/attenuation level may be directly related to a number ofprogrammable factors/parameters, as well as the current on the VBusline, dynamically overcoming any GND offset changes due to increasedcurrent being drawn by the device attached to the Host (e.g. to Host302), for example. Block 112 may be used to provide under-voltagelockout (UVLO) and over-voltage lockout (OVLO) to protect the attacheddevice in case there is an over-voltage or under-voltage condition onthe supply line Vs. Furthermore, an overcurrent limit (OCL) value may beprogrammed/set in block 104 to specify a maximum current level allowedto be drawn by the attached device, and exceeding this limit may resultin the generation of an overcurrent event.

For example, referring back to FIG. 5, Host 302 may be designedaccording to the embodiment shown in FIG. 1. In that case, USB Host 302may include Host functionality 202, port power controller 120, andphysical USB Connector 204. Therefore, USB connector 204 may representthe outputs provided by USB Host 302, including the data signals D+ andD−. Assuming that the device (again, e.g. Function 306 via Hub 304)draws, for example, 1 A in charging mode, what originates as a 400 mVsignal from the Host function 202 may be boosted to e.g. 650 mV on theD+ line and 250 mV on the D− line by a level shifter in boost module116, based on a measured value (of 1 A, in the example case) of thecurrent flowing in the VBus line. Accordingly, the signal at the deviceside (e.g. at 304) may now appear as a differential voltage of 400 mV,which is no longer close to the very limits of the USB specification. Itshould be noted that the voltage values above are provided as examplesfor illustration purposes, and the boost level(s) may be configurable,e.g. in boost block 116 and/or in block 104 as desired. It shouldfurther be noted, that in the example provided above the voltage levelson the D+ and D− line are boosted, but the structures disclosed in FIG.2 may equally be operated to attenuate the signals as may be required,by generating the appropriate control signal(s) 114.

FIG. 7 shows a flow diagram of one embodiment of a method for overcominglimited common-mode range on a differential bus. In 702 a supply voltageis provided over a supply line to a device. In 704 data is received overa differential data bus. In 706 the current drawn on the supply line ismeasured, and a control signal based on the measurement results isgenerated in 708. In 710 the voltage level of the received data isadjusted using the control signal. Therefore, the voltage level of thereceived data may be adjusted according to the level of the currentdrawn on the supply bus by the device. In 712 the adjusted voltage leveldata is provided for transmission to the device.

When receiving data over the differential data bus (704), the data maybe received over a pair of differential data lines, and the voltagelevel of the data on each data line of the pair of differential datalines may be adjusted for adjusting the voltage level of the data.Furthermore, when adjusting the voltage level of the data (710) on eachdata line of the pair of differential data lines, the voltage level ofthe data on each data line may be adjusted by an equal amount. In 714,the adjusted voltage level data is finally transmitted to the deviceover an outgoing differential data bus. It should be noted that the flowdiagram in FIG. 7 shows data flow to the device, but the same data flowmay apply to data received from the device, and the boost level for thetransmit and receive path may be individually programmable, for examplein blocks 104 and 116, and may allow for both a programmable boost andprogrammable attenuate, should there be a need to attenuate the datasignals.

Further modifications and alternative embodiments of various aspects ofthe invention may be apparent to those skilled in the art in view ofthis description. Accordingly, this description is to be construed asillustrative only and is for the purpose of teaching those skilled inthe art the general manner of carrying out the invention. It is to beunderstood that the forms of the invention shown and described hereinare to be taken as embodiments. Elements and materials may besubstituted for those illustrated and described herein, parts andprocesses may be reversed, and certain features of the invention may beutilized independently, all as would be apparent to one skilled in theart after having the benefit of this description of the invention.Changes may be made in the elements described herein without departingfrom the spirit and scope of the invention as described in the followingclaims. For example, while the specific embodiments provided hereinfocus on the established USB standard, other embodiments may equally bedesigned to monitor and measure currents drawn on a supply line and orsupply bus, and the levels of the differential data may be adjustedaccording to the current drawn on the supply line by a device alsoexpecting to receive the data.

I claim:
 1. A port power controller (PPC) comprising: a supply busconfigured to provide a supply voltage; a data bus distinct from thesupply bus and configured to carry data wherein the data bus incomprised of one or more data lines; a measurement unit configured tomeasure the current drawn on the supply bus; and a level shifterconfigured to adjust one or more voltage levels of the one or more datalines, wherein the one or more voltage level adjustments uniformlyadjust the voltage levels of the one or more data lines by the sameamount, wherein the one or more voltage level adjustments are madeaccording to a value of the current measured by the measurement unit inorder to compensate for the effects of the supply voltage on the voltagelevels of the data lines that comprise the data bus, and wherein the oneor more voltage level adjustments are proportional to changes in themeasured current drawn on the supply bus.
 2. The PPC of claim 1, whereinthe data bus comprises a Universal Serial Bus (USB) D+ data line and aUSB D− data line.
 3. The PPC of claim 1, further comprising themeasurement unit configured to generate a control signal based on themeasured value of the current drawn on the supply bus wherein thecontrol signal is used by the level shifter to adjust the one or morevoltage levels of the one or more data lines that comprise the data bus.4. The PPC of claim 1, wherein in adjusting the voltage level of data onthe data bus the level shifter is configured to uniformly increase ordecrease the one or more voltage levels of the one or more data lineswherein the determination to increase or decrease the voltage levels isbased on the direction of the data flow on the data bus.
 5. A method forovercoming limited common-mode range on a differential data bus, themethod comprising: providing a supply voltage over a supply bus to adevice; receiving data on an incoming differential data bus comprised ofone or more data lines; measuring the current drawn on the supply bus;adjusting one or more voltage levels of the one or more data lines,wherein the one or more voltage level adjustments uniformly adjusts thevoltage levels of the one or more data lines by the same amount, whereinthe one or more voltage level adjustments are made according to themeasured value of the current drawn on the supply bus in order tocompensate for the effects of the supply voltage on the voltage levelsof the one or more data lines that comprise the data bus, and whereinthe one or more voltage level adjustments are proportional to changes inthe measured supply bus current; and providing data for transmission tothe device at the adjusted voltage levels over the outgoing differentialdata bus.
 6. The method of claim 5, further comprising: generating acontrol signal based on said measuring; wherein said adjusting comprisesadjusting the one or more voltage levels of the data using the controlsignal.
 7. The method of claim 6, wherein said adjusting the one or morevoltage levels of the one or more data lines using the control signalcomprises; providing the control signal to a level shifter; and thelevel shifter increasing the voltage level of the one or more data linesaccording to the control signal.
 8. The method of claim 5, wherein saiddifferential data lines comprise a D+ data line and a D− data line,wherein said adjusting comprises adjusting the voltage level of the dataon the D+ data line and the voltage level of the data on the D− dataline.
 9. The method of claim 5, wherein the data is Universal Serial Bus(USB) data, the method further comprising transmitting the supplyvoltage and the adjusted voltage level USB data over a USB to a USBdevice.
 10. A Universal Serial Bus (USB) Host device comprising: a USBHost function configured to provide USB data signals over differentialdata lines; a port power controller (PPC) coupled to the USB Hostfunction, and configured to: provide a supply voltage on a supply bus;receive the USB data signals on the differential data lines; adjust thevoltage levels of the USB data signals on the differential data lines,wherein the voltage level adjustments uniformly adjusts the voltagelevels of the differential data lines by the same amount, wherein thevoltage level adjustments are made according to a current drawn on thesupply bus in order to compensate for the effects of the supply voltageon the voltage levels of the differential data lines, and wherein thevoltage level adjustments to the data signals are proportional tochanges in the measured supply bus current; provide the supply voltageat a supply port; and provide the USB data signals at the adjustedvoltage levels at a data port; and a measurement unit of the PPCconfigured to measure the current drawn on the supply bus; USB connectorcoupled to the PPC and configured to couple to a USB to transmit thesupply voltage and the adjusted USB data signals.
 11. The USB Hostdevice of claim 10, wherein the PPC is further configured to; generate acontrol signal based on the measured current and use the control signalto adjust the voltage levels of the received USB data signals.
 12. TheUSB Host device of claim 10, wherein in adjusting the voltage levels ofthe received USB data signals the PPC is configured to boost the voltagelevels of the received USB data signals wherein the determination toboost the voltage levels is based on the direction of the data flow onthe data bus.
 13. A system comprising: a serial bus comprising a pair ofdifferential data lines and a power supply line; a device coupled to theserial bus; and a host coupled to the serial bus, and configured to:provide a supply voltage on the power supply line; measure the value ofthe current drawn on the supply line; provide a pair of differentialdata signals over the differential data lines; adjust the voltage levelsof the differential data signals, wherein the voltage level adjustmentsuniformly adjusts the voltage levels of the differential data lines bythe same amount, wherein the voltage level adjustments are madeaccording to the measured value of the current drawn on the power supplyline by the device in order to compensate for the effects of the supplyvoltage on the voltage levels of the differential data lines, andwherein the voltage level adjustments are proportional to changes in themeasured supply bus current; and transmit the pair of differential datasignals at the adjusted voltages to the device via the pair ofdifferential data lines of the serial bus.
 14. The system of claim 13,wherein the serial bus is a Universal Serial Bus (USB), and the pair ofdifferential data lines comprise a USB D+ line and a USB D− line. 15.The system of claim 13, wherein the host is further configured to:generate a control signal based on the measured current; and adjust thevoltage levels of the generated pair of differential data signals usingthe control signal.
 16. The system of claim 14, wherein the hostcomprises a level shifter, wherein the host is configured to control thelevel shifter with the control signal to adjust the voltage levels ofthe generated pair of differential data signals.